Alarm system having phase-sensitive bridge circuit

Abstract

An alarm system for detecting a dangerous condition such as the increase in content of or the presence of toxic or inflammable gas in the air and giving a warning signal. The alarm system comprises an AC power supply, a full-wave rectifier connected to the AC power supply and rectifying alternating current from the AC power source into direct current. An AC bridge circuit connected to the AC power supply and includes a sensor element at least in one branch. An AC operational amplifier is supplied with power from the full-wave rectifier and receives an input signal which is the output signal delivered from the bridge circuit through a DC blocking capacitor. A thyristor is connected in parallel with the AC power supply, with its control gate connected to the output of the operational amplifier, and its main electrodes in series with an alarm indicator actuable when the thyristor is triggered into the conduction state. Such triggering occurs when the output signal voltage from the operational amplifier reverses its phase by 180°, i.e., when the resistance of the sensor element varies when a dangerous condition exists.

What is claimed is: 
     
       1. An alarm system comprising (a) an AC power supply circuit,   (b) a full-wave rectifier for effecting full-wave rectification of the alternating current supplied from said AC power supply circuit,   (c) an AC bridge circuit which is supplied with the power from said AC power supply circuit and which includes a sensor element in at least one branch of the bridge thereof,   (d) a DC blocking capacitor having one terminal connected to an output terminal of said bridge circuit,   (e) an operational amplifier which is supplied with power from said full-wave rectifier and receives the output signal from said AC bridge circuit through said DC blocking capacitor, an input terminal of said amplifier being connected to the other terminal of said blocking capacitor,   (f) a thyristor having main electrodes connected to said AC power supply circuit and a control electrode for receiving the output signal from said operational amplifier, and   (g) an alarm indicator coupled to at least one of said main electrodes of said thyristor and actuable when said thyristor is triggered into the conduction state by said operational amplifier output signal when the phase of the AC output signal from said AC bridge circuit is reversed by 180° due to the variation in resistance of said sensor element under abnormal conditions.   
     
     
       2. An alarm system as set forth in claim 1 wherein a differentiating capacitor is connected to the output of said operational amplifier. 
     
     
       3. An alarm system as set forth in claim 1 wherein a smoothing circuit is connected to the output of said full-wave rectifier. 
     
     
       4. An alarm system as set forth in claim 1 wherein a differentiating capacitor is connected to the output of said operational amplifier; and a smoothing circuit is connected to the output of said full-wave rectifier.